Add a use case to the manual for aerodynamic forces on external flow objects

[lcarasik]

It would be useful to add a use case of calculating drag and lift on an external flow object.

subroutine torque_calc(scale,x0,ifdout,iftout)
Computes torque about the point x0. Here scale is a user supplied multiplier so that the results may be scaled to any convenient non-dimensionalization. Both the drag and the torque can be printed to the screen by switching the appropriate ifdout(drag) or iftout(torque) logical.
Ref: http://nek5000.github.io/NekDoc/appendix.html?highlight=drag

Reference Discussion: https://groups.google.com/d/msgid/nek5000/68bdbfa5-e1cb-41b1-95c1-dff252bd1b3b%40googlegroups.com?utm_medium=email&utm_source=footer